The optical disks known as CD (Compact Disc), DVD (Digital Versatile Disc), etc. have spread widely as compact information recording media with large capacities. As a recording film of the optical disk on which information is rewritable, one that uses a phase change material has become commercially practical and the one enables information to be recorded, erased, or overwritten by means of reversible phase change between a crystal phase and an amorphous phase of the material.
In this type of rewritable optical disk, recording of information is made possible by irradiating high-power laser light onto the recording film and forming amorphous recording marks. It utilizes a fact that, when light is irradiated onto a recording film, its reflectivity is different between the recording mark of an amorphous material and a portion that is a crystalline layer and is other than the recording mark. On the other hand, reproduction of information is made possible by irradiating laser light of such a low power as does not change the recording film and detecting a difference of the reflectivity between the recording mark (amorphous phase) and the portion other than the mark (crystal phase). Moreover, erasing of information is made possible by irradiating laser light of a medium power between the high-power laser light at the time of information recording and the comparatively low-power laser light at the time of information reproduction. On the other hand, overwriting information becomes possible by irradiating a combination of the high-power light and the medium-power light.
Now, looking specifically at the phase change type optical disk, if it is subjected to information reproduction with laser light of an excess power, or if the same portion is repeatedly reproduced many a time, there occurs a phenomenon that a part of the amorphous phase returns to the crystal phase, namely a part of the mark are erased. By this phenomenon, some of the recording marks deteriorate and become hard to recognize, and accordingly a reproduction error tends to occur.
Especially, in the case of a combination of a high-density optical disk, such as the BD disk (Blue-ray Disc) that has been put in practical use lately and an optical system that uses a short-wavelength blue light source (wavelength is approximately 400 nm) as a light source, since a light spot size is small and the power density of the spot is large, recording mark deterioration during reproduction tends to occur easily as compared with a case of a red light source even when the information is reproduced using the same power; therefore, the read power cannot be made larger than that of the case of the red light source. The BD disk is described in detail in White Paper: Blu-ray Disc Format—General.
Practicably, since the disk is required to allow the information to be reproduced repeatedly for 106 to 108 times or more, it is often the case that an upper limit of the read power is specified for each disk medium or specified collectively by a standard in order to guarantee a predetermined number of reproducible times. In addition, from the viewpoint of protection of recorded information, when reproducing an optical disk in a drive device, an upper limit of the read power in the case of using a read power of, for example, a blue light source must be strictly observed.
However, since laser lights of short wavelengths like a blue light source have small photoelectric conversion efficiencies in photodetectors and the signal to noise ratio of a readout signal found when information is reproduced with the laser light of a short wavelength is small, a reproduction error tends to be produced. Therefore, from the viewpoint of reliability (low error rate) of information reproduction, it is desirable that the read power is as large as possible in order to obtain a large signal to noise ratio of the readout signal.
In order to establish compatibility between the above-mentioned two requirements, that is, protection of recorded information and reliability of information reproduction at the time of reproducing recorded information, the read power must be controlled with high accuracy. As the conventional method for controlling the read power, for example, there has been used a method for controlling the magnitude of a current flowing in a laser diode of a light source so that the laser light outputted from the objective lens may be kept at a predetermined magnitude. This method configures the system such that a part of the laser light outputted from a laser diode is divided in a halfway, and is made to enter into a photodetector for exclusive use (front monitor), and it controls the drive current of laser diode so that the output current of the front monitor may be maintained at a predetermined magnitude.